> 80% is propulsive efficiency from the shaft to useful thrust, there's a lot more energy lost between the fuel input and shaft output (thermodynamic efficiency

> 80% is propulsive efficiency from the shaft to useful thrust, there's a lot more energy lost between the fuel input and shaft output (thermodynamic efficiency). The thermodynamic efficiency is where most energy is wasted. For small turboprop engines, this is only 25-30%. Ah, I see, so "propellant" in propulsive energy refers to the ignited fuel/air mixture, not accounting for thermal losses caused by fuel ignition. Given that, the efficiency case for H2 fuel-cell flight is even better than I expected (and it was already pretty good). I'm guessing they also don't lose efficiency to noise and vibration.